Photovoltaic cells made with silicon wafers for terrestrial applications usually comprise a p-type silicon wafer. One surface of the wafer is “doped” (usually with phosphorus at temperatures of 900–1000° C.) to make it n-type and produce the basic p-n junction which is the active component of the solar cell. Metal contacts are then applied to the front and rear surfaces (the n and p surfaces) to enable the photo-generated current to be carried from the cell. In the case of the front contact the metal is typically deposited as an open grid pattern to allow light to be absorbed by the exposed silicon surface. The front grid can be deposited in embedded grooves to reduce grid shading losses. This type of solar cell is often known as a Buried Contact solar cell (also referred to as a Laser Grooved Buried Grid—LGBG solar cell). The basic cell is disclosed in EP 156366A.
The process for fabricating such a cell typically includes the following initial steps:
1) in the case of monocrystalline silicon wafers with [100] crystal orientation, etching of the silicon surface in a caustic solution to form random pyramids;
2) doping the top surface of a p-type silicon wafer with a Group V element, typically phosphorus, to produce the n+ layer;
3) adding a top surface coating of silicon nitride, to act as an antireflection layer and also as a dielectric (non-conducting) layer in order to prevent metal plating on unwanted regions of the top surface;
4) cutting grooves into the surface of the wafer into which metal will be plated. The grooves are typically cut using a laser but may be chemically etched, plasma etched, or mechanically formed using a diamond saw;
5) doping the exposed p-type silicon surface in the grooves n-type with a Group V element, typically phosphorus;
6) treating the cell to provide a back surface field (BSF), by doping the rear surface with an electron “acceptor” such as aluminium or boron;
7) plating metal contacts into the grooves to provide an electrically conducting front contact, and also simultaneously into the rear surface in order to provide an electrically conducting back contact.
Following the above steps, it is necessary to electrically isolate the edges of the wafer. This step is not relevant to the present invention.
A back surface field (step 6) on a crystalline silicon solar cell serves to boost the efficiency of the cell by repelling photo-generated minority charge carriers (electrons in p-type silicon) away from the rear surface where they would otherwise recombine with majority carriers (holes in p-type silicon) before they can be made to do useful work in an external circuit. In high efficiency laboratory cells the BSF is often formed by depositing a film of aluminium by physical vapour deposition and heating to temperatures in excess of 1050° C. (typically over 1100° C.). However, these cells typically achieve a BSF due to special circumstances which are not appropriate for low-cost commercial cell fabrication:                The aluminium thickness used is typically equal to or greater than 2 μm and it has been reported that this is a minimum thickness required for the formation of an effective BSF.        The silicon wafers used are made by Floating Zone (FZ) melting and contain low concentrations of oxygen. These wafers can be heated to high temperatures for extended periods without destroying their electronic performance. For reasons of economy however, commercial cells must be made from Czochralski (CZ) silicon wafers which contain dissolved oxygen impurities and suffer performance degradation when heated above 1000° C. for extended periods.        
In commercial solar cells, the BSF is usually formed by printing a thick film (˜20 μm) of aluminium or spin-coating a film of a boron containing compound onto the rear surface of the silicon wafer, and heating the film to temperatures in the range 800–900°C. for aluminium (1000–1100° C. for boron), to incorporate the metal at a concentration of about 100 ppm. As a result these cells use a relatively large quantity of aluminium. In cells incorporating a BSF through the use of a thin film (˜2 μm) aluminium layer, the rear surface of the wafer typically incorporates a thin dielectric layer of silicon oxide or silicon nitride. In addition to passivating the rear surface, this dielectric layer serves as a barrier to prevent phosphorus atoms from doping the rear surface (during formation of the front junction of the cell or the doping of the groove) and preventing good BSF formation. Doping of the aluminium atoms through this layer is achieved by either high temperature diffusion (in the case of silicon oxide barrier) or through the formation of apertures in the dielectric layer to produce a localised BSF. But this adds additional cost and complexity to cell fabrication.
As a result, there is a need for a method of applying a BSF to a silicon solar cell which is more efficient than that used for commercial cells, and which can also lead to improved cell efficiency.